1. Field of the Invention
The present invention relates to an oxide superconductor thick film containing Bi, Pb, Sr, Ca and Cu having a high critical current density and a high adhesiveness to an oxide substrate or an oxide base, and a method for manufacturing the same.
2. Description of the Related Art
An oxide substrate or an oxide base such as MgO, alumina or YSZ, or a metal substrate or a metal base such as Ag, Au, Pt or Ni is formed with an oxide superconductor in thick film form to function, so as to devise a variety of applied products.
As a method of forming this oxide superconductor into film form, a technique is tried in which an oxide superconductor powder, to which an appropriate organic binder is added, is made into paste form, thereafter applied to the substrate or the base using the screen printing method, the doctor blade method, the spray method, or the like, and burned to thereby form a polycrystalline oxide superconductor thick film.
This technique of forming the oxide superconductor thick film using an oxide superconductor paste has a quite low manufacturing cost, that is, an advantage because of no need for an expensive single crystal substrate and a large-scale and expensive apparatus requiring a high vacuum system represented by PVD, CVD or the like, and thus it is considered as the technique closest to practical use.
When an application of this oxide superconductor thick film to a practical product is considered, an adoption of a thick film containing (Bi, Pb)2+aSr2Ca2Cu3Oz (generally, 0<a<0.5) (hereinafter, described as Bi2223 based thick film) as an oxide superconductor thick film material is prospective from two perspectives of required superconducting characteristics and manufacturing costs including raw materials and manufacturing processes.
A method of forming the Bi2223 based thick film is explained below.
First, a Bi based superconductive paste is applied to an oxide base such as MgO, alumina, or YSZ using the screen printing method, the doctor blade method, the spray method, or the like. At this stage, an oxide superconductor powder in the Bi based superconductive paste applied to the base has a (Bi, Pb)2Sr2Ca1Cu2Oz phase (hereinafter, described as Bi2212 phase) whose critical temperature is between Tc and approximately 80 K as a main phase, and not the (Bi, Pb)2+aSr2Ca2Cu3Oz phase (hereinafter, described as Bi2223 phase) whose critical temperature is between Tc and approximately 110 K. This oxide superconductor powder is a multi-phase which also includes an intermediately generated phase such as Ca2PbO4, CaCuO2, or CuO.
Then the Bi based superconductive paste is heat treated, so that a reaction occurs between the (Bi, Pb)2Sr2Ca1Cu2Oz and the intermediately generated phase to make the (Bi, Pb)2+aSr2Ca2Cu3Oz, and thus the Bi2223 phase is generated from the Bi2212 phase and the intermediately generated phase. As a result, an oxide superconductor thick film containing the Bi2223 phase having a high critical temperature is formed on the oxide base.
Although it is important to have a high critical temperature when the oxide superconductor thick film is applied to a practical product, it is also required to have a high critical current density (hereinafter, described as Jc). Further, when the oxide superconductor thick film is applied to a practical product, a Jc of more than 5,000 A/cm2 is required.
Generally, to attain a Jc of more than 5,000 A/cm2 using a thick film containing the above-mentioned Bi2223 phase (hereinafter, described as Bi2223 thick film), plate crystals having large ab faces and short c axes, which are peculiar to the Bi based oxide superconductor, are appropriately oriented so as to align their ab faces, on which a superconducting current easily flows, in the conducting direction. Consequently, a compressing operation using a CIP (Cold Isostatical Press), or an HIP (Hot Isostatical Press) is adopted for aligning the plate crystals in the conducting direction.
However, even with the high Jc attained by using such compressing operation, when a substantial cross-sectional area of the thick film on which the superconducting current flows was small, an adequate net value of the superconducting current cannot be attained, and thus it is not suitable for practical use. Therefore, it is necessary to design a Bi2223 thick film in a way that the cross-sectional area for an adequate critical current value (Ic) is assured.
Accordingly, following processes are normally taken in order to form an oxide superconductor thick film having a high Jc and a high critical current value on a base.
The patent document 1 is referred as an example.
1. A Bi based oxide superconductor paste is applied to an appropriately selected oxide base to have more thickness than a predetermined film thickness.
2. A first burning is conducted to the oxide base having the Bi based oxide superconductor paste applied thereon.
3. After the first burning, the oxide base is placed in a CIP and compressed therein.
4. The burning of the process 2 and the compressing of the process 3 are repeated for an appropriate number of times. After the last burning is completed, the oxide base having the Bi2223 thick film applied thereon is obtained.
Patent document 1: Japanese Patent Application Laid-open No. 2001-358298
In the above-described method of forming a Bi2223 thick film related to the prior art, a peeling of a thick film from an oxide base occurs frequently in the middle of a film-forming process, which lowers an yield of a process and thus decreases productivity. This peeling phenomenon commonly occurs during a burning or after a CIP, especially during a first burning or after a first CIP. The present inventors conducted a study on this peeling phenomenon of the thick film from the oxide base, and then found following factors.
First, the peeling during a burning attributes to:
1. a difference in a coefficient of thermal expansion (linear expansion) between an oxide base and a superconductor,
2. a cubical volume change due to an expansion of a whole thick film that occurs concurrently with a transformation of a crystal structure to generate a high Tc phase from a low Tc phase and an intermediately generated phase.
In particular, an effect of the factor 2 is dominant during a first burning, and thus it is considered to be a main cause for the frequent occurrence of peeling during a burning.
Next, the peeling after a CIP attributes to:
3. a phenomenon of force in such a direction as to peel a thick film from an oxide base. This force is generated as a residual stress inside the thick film when the thick film and the oxide base having the thick film formed thereon are compressed with a predetermined isostatic pressure (0.5 to 3.0 ton/cm2), and thereafter, the residual stress is released to take the above effect when the thick film and the oxide base are decompressed. In addition, this phenomenon of force prominently affects an oxide base in cylindrical shape.
The present inventors conducted an analysis of the Bi2223 thick film formed on a base related to the prior art. The results of this analysis are explained below with reference to FIG. 7. FIG. 7 is an optical microphotography of a cross section of the Bi2223 thick film formed on an MgO base related to the prior art.
In FIG. 7, a reference numeral 51 designates a cross section of the MgO base, a reference numeral 59 designates the cross section of the Bi2223 thick film related to the prior art, and a reference numeral 53 designates an interface between the MgO base 51 and the Bi2223 thick film 59. In a detailed observation of FIG. 7, it is found that a fracture surface 58 is generated in an area of approximately 2 μm to 10 μm from the interface 53 in the Bi2223 thick film 59. The present inventors further conducted similar analysis to a large number of Bi2223 thick films related to the prior art, and recognized that a fracture surface exists in an oxide superconductor thick film not only in a case that an oxide superconductor thick film peels off from an oxide base in the middle of a manufacturing process, but also in a case that the thick film does not peel off from the oxide base.
When a fracture surface is relatively large, an oxide superconductor thick film peels off from an oxide base during a burning or a CIP compression in the middle of a manufacturing process. When the fracture surface is small, the oxide superconductor thick film does not peel off from the oxide base in the middle of a manufacturing process. However, the fracture surface develops every time a thermal or a mechanical shock is applied to the oxide superconductor thick film, which finally leads to the peeling of the oxide superconductor thick film.